The present invention relates to ceramic investment shell molds and their manufacture for casting metals and alloys.
In casting superalloy gas turbine engine blades and vanes using conventional equiaxed and directional solidification techniques, ceramic investment shell molds with or without a ceramic core therein are filled with molten metal or alloy that is solidified in the mold. The ceramic shell mold is made by the well known lost-wax process where a fugitive (e.g. wax) pattern of the blade, vane or other article to be cast is repeatedly dipped in a ceramic slurry, drained of excess slurry and then stuccoed with ceramic particulates, such as ceramic sand (stucco) to build up the shell mold wall thickness to a desired value. The pattern then is selectively removed from the shell mold by thermal or chemical dewaxing techniques, and the green mold is fired to develop adequate mold strength for casting. U.S. Pat. Nos. 5,335,717 and 5,975,188 describe a typical lost-process sequence to make ceramic investment casting shell molds.
Current lost-wax mold-making processes employ water-based ceramic slurries and low temperature wax patterns. Production of a ceramic shell mold around such a pattern typically takes more than forty hours.
Pattern materials such as wax usually are used in the lost-wax process at pattern temperatures less than about 78 degrees F. because the wax pattern melts or softens at sustained wax temperatures above 80 degrees F., resulting in pattern distortion. Moreover, when the pattern is coated with a layer of water based ceramic slurry, the temperature of the pattern drops as it provides heat of evaporation. This temperature decrease not only reduces the subsequent drying rate of the ceramic slurry, but also results in pattern contraction during cooling and subsequent expansion when the wax pattern warms up again, the latter unfortunately coinciding with the slurry layer drying and becoming more rigid. Shell mold cracks can be initiated by the thermal expansion mismatch between the relatively high expansion wax pattern and relatively low expansion shell layer when the wax temperature returns back to ambient temperature before the next dipping/stuccoing step of the lost-wax process.
High temperature, low humidity drying air (e.g. 1-10% relative humidity air) and high speed flowing drying air conditions frequently are used in the lost-wax process after each dipping/stuccoing step to speed shell mold manufacture and can result in larger temperature drops in a first few minutes after dipping the pattern in the slurry. Cracking of the slurry layers can occur during the shell mold building process and also during the pattern removal operation as a result of thermal expansion mismatch of the pattern and the shell. U.S. Pat. No. 4,114,285 describes drying conditions to make ceramic shell molds to speed and improve the mold production process.
An object of the present invention is to substantially reduce the processing time to make a ceramic shell mold.
Another object of the present invention is to substantially reduce shell mold cracking during the mold building steps and pattern removal operation.
An embodiment of the present invention provides a method of making a ceramic investment shell mold wherein ceramic particulates, such as for example sand or stucco, heated to superambient temperature are applied to at least some ceramic slurry layers on the pattern to reduce pattern temperature fluctuations during the mold building process.
In a particular embodiment of the present invention, at least some ceramic slurry and heated stucco layers applied on the pattern are initially dried for a time in relatively low humidity flowing air at an air temperature above a pattern thermal degradation temperature (e.g. at a temperature about 80-95 degrees F. for a wax pattern) and then subsequently dried for a time in relatively low humidity flowing air at an air temperature below the degradation temperature (e.g. not exceeding about 78-80 degrees F. for a wax pattern).
Ceramic investment shell molds can be built-up in times typically less than 10 to 20 hours depending on cast component, and thus on mold complexity, by practice of the invention with substantially reduced incidence of shell mold cracking during the shell mold building process and subsequent pattern removal operation.
The above objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.